Optical fiber networks are used in a variety of applications, such as optical telecommunication and data transmission systems. Optical fiber networks use optical fibers as transmission lines for carrying light signals. The light signals propagate down the fiber from one location to another, analogous to electrical signals traveling down a wire or cable from one location to another. Optical fibers are used in a variety of applications, such as metro access local loops and “long haul” transmission lines. Long haul transmission lines might carry signals between cities or across oceans, for example.
Optical fibers can carry a single channel, or many channels can be multiplexed onto a single fiber. Multiplexing is way of increasing the information-carrying capacity through an optical fiber. There are various ways to multiplex signals on an optical fiber or other type of transmission line, including time division multiplexing and wavelength division multiplexing (“WDM”). In a WDM system, a number of wavelength channels are carried on a single optical fiber. A channel is typically defined as a frequency (wavelength) of light that is modulated to carry information. Optical WDM networks typically allocate a portion of the spectrum about a center frequency of the nominal channel wavelength for signal transmission. For example, channels might be spaced 100 Glz apart with ±12.5 GHz on either side of the channel center frequency in a particular system, thus providing the channel with a “width” of 25 GHz. Channel spacing of 100 GHz or less is commonly referred to as dense wavelength division multiplexing (“DWDM”) Other systems may require or allow a narrower or wider channel widths or spacings.
It is typically very expensive to install a WDM network, and therefore it is desirable to use the installed system as efficiently as possible. DWDM is a technology enabling increased capacity of fiber networks without needing to install additional fiber cables. However, it is not a simple matter to merely add more channels onto an existing fiber network. For example, a light pulse in a digital transmission system is typically “spread” or dispersed as it travels along an optical fiber and through components of the optical communication network, and dispersion may affect the new channels beyond acceptable limits. Similarly, a light signal loses strength as it propagates down an optical afiber, and repeater stations are typically required at regular intervals (e.g. every 100 km) to boost (amplify) the signal. Adding channels may affect the gain available per-channel at each repeater station, and thus require more powerful amplifiers or additional amplifier states.
One conventional method routes the entire spectrum (bandwidth) carried by the fiber into a broad-band amplifier and amplifies all signals. Unfortunately, such amplifiers generally do not amplify all channels equally. If amplifiers with similar characteristics are used in a chain of repeaters, the amplitude difference between channels accumulates. Some sort of amplitude equalization is typically required. A further problem is that the amplifier is generally optimized for a certain input power and bandwidth (“gain bandwidth”). Thus, adding additional channels essentially dilutes the power available for each channel or may fall outside of the gain bandwidth, and if the added channels are outside of the original design bandwidth, the amplifier might not supply sufficient gain for these new channels. Adding additional amplifiers to accommodate additional channels might require “breaking” the transmission line, disrupting all communication traffic while the new amplifier is installed.
Another method routes the optical signal to a multiplexer and de-multiplexes the signal into channels or multi-channel segments of the transmission spectrum. Each segment of the spectrum is amplified and routed to a multiplexer to combine all the segments back onto the transmission line, thus allowing amplifiers to be optimized for a particular of the transmission spectrum. However, multiplexers typically have a fixed “fan-out”, thus any expansion of the network must be anticipated at the time the demultiplexer/multiplexer is installed. Unfortunately, it is not always possible to anticipate how a network might be expanded, or how many channels might be added. Therefore, installation of a multiplexer with insufficient or inappropriate fan-out might present the network operator with the dilemma of not utilizing the installed optical transmission lines at their full capacity, or disrupting all signal traffic to install a new multiplexer system.
Furthermore, installing a multiplexer capable of handling the total number of anticipated channels requires an initial expense for capacity that is not used for some time. For example, an optical transmission line might have an anticipated capacity of 40 channels, whereas only 8 channels will initially be occupied with optical data transmission. A 1×40 de-multiplexer and multiplexer would typically be installed, even though only 8 channels will initially be used. If the channel capacity of the optical transmission line changed to 80 channels, the network operator would be faced with the choice discussed in the preceding paragraph. If the channel capacity or definition were changed again, the same dilemma would be presented. The dynamic growth of optical networks and capabilities of the transmission lines and their components makes accurately predicting how an optical network will expand difficult, at best.
Thus, expanding an optical fiber network to accommodate additional channels presents many challenges. It would be desirable to provide a system that allowed upgrades without disrupting all traffic on an optical transmission line, and without needing to predict the manner in which the network might develop. It would be further desirable that upgrade capability include the ability to add additional channels in the transmission spectrum. It would also be desirable that such a system provide capability for signal improvement, such as dispersion compensation or level control, and avoid excessive insertion loss.